BACKGROUND: Interventions for curing most diseases and save lives of pregnant and delivering women exist, yet the power of health systems to deliver them to those in most need is not sufficient. The aims of this study were to design a participatory antenatal care (ANC) strengthening intervention and assess the implementation process and effectiveness on quality of ANC in Jimma, Ethiopia. METHODS: The intervention comprised trainings, supervisions, equipment, development of health education material, and adaption of guidelines. It was implemented at public facilities and control sites were included in the evaluation. Improved content of care (physical examinations, laboratory testing, tetanus toxoid (TT)-immunization, health education, conduct of health professionals, and waiting time) were defined as proximal project outcomes and increased quality of care (better identification of health problems and increased overall user satisfaction with ANC) were distal project outcomes. The process of implementation was documented in monthly supervision reports. Household surveys, before (2008) and after (2010) intervention, were conducted amongst all women who had given birth within the previous 12 months. The effect of the intervention was assessed by comparing the change in quality of care from before to after the intervention period at intervention sites, relative to control sites, using logistic mixed effect regression. RESULTS: The continued attention to the ANC provision during implementation stimulated increased priority of ANC among health care providers. The organizational structure of the facilities and lack of continuity in care provision turned out to be a major challenge for implementation. There was a positive effect of the intervention on health education on danger signs during pregnancy (OR: 3.9, 95% CI: 2.6;5.7), laboratory testing (OR for blood tests other than HIV 2.9, 95% CI: 1.9;4.5), health problem identification (OR 1.8, 95% CI: 1.1;3.1), and satisfaction with the service (OR: 0.4, 95% CI: 0.2;0.9). There was no effect of intervention on conduct of health professionals. The effect of intervention on various outcomes was significantly modified by maternal education. CONCLUSION: The quality of care can be improved in some important aspects with limited resources. Moreover, the study provides strategic perspectives on how to facilitate improved quality of ANC.